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| Titel |
Ammonium in crustal and subduction zone fluids |
| VerfasserIn |
Anke Watenphul, Christian Schmidt |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250046881
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| Zusammenfassung |
| Ammonium, NH4+, can be an important constituent in many crustal and subduction zone
fluids, but is usually overlooked because of analytical difficulties. Large amounts of NH4+
are generated by the decomposition of organic material during diagenesis. Ammonium is
readily mobilized in hydrothermal processes. Geothermal brines can contain more than 1000
ppm NH4+. Hydrothermal deposits often display substantial ammonium halos
with up to 25000 ppm NH4+ in the host rocks (Ridgway et al., 1990). In minerals,
ammonium is mainly incorporated into potassium silicates, some of which can transport
NH4+ to eclogite facies conditions (Busigny et al., 2003), or even into the mantle
(Watenphul et al., 2009). Usually, a large fraction of the incorporated ammonium is
released upon progressive metamorphism (Mingram and Bräuer, 2001) because
NH4+ strongly fractionates into the fluid relative to mica and feldspar (Pöter et al.,
2004).
The behaviour of ammonium in aqueous fluids was experimentally studied to 600 Ë C and
about 1.3 GPa in a hydrothermal diamond-anvil cell. The observed Raman spectroscopic
detection limit for NH4+ in aqueous solutions is about 0.2 molal, which explains the
disregarding in fluid inclusions. However, the Raman spectra also reveal that the addition of
ammonium chloride causes a strong reduction of the dynamic three-dimensional network of
water. This points to a substantial decrease in the water activity even if only quite small
amounts of NH4Cl (< 1 molal) are present. Experiments on the assemblages NH4Cl
solutions plus quartz or quartz + kyanite + K-feldspar ± muscovite show that the silica
solubility is significantly lower than that in equimolal NaCl solutions. This indicates that
ammonium causes an even stronger decrease in the activity of water in chloridic solutions
than sodium.
At temperatures above 300 Ë C, a significant fraction of NH4+ is converted to ammonia,
NH3, indicating a large shift towards acidic conditions. The NH3/NH4+ ratio increases with
temperature and decreases with pressure. This implies that more ammonium should be
retained in K-bearing minerals coexisting with chloridic fluids upon high-pressure
low-temperature metamorphism.
Experiments on the kinetics of the reaction K-feldspar + kyanite + water = muscovite +
quartz show a much faster proceeding in experiments with NH4Cl solutions than in
comparable experiments with water. This is attributed to the enhancement of the rate-limiting
alumina solubility. The nucleation and growth of mica on the expense of K-feldspar and
NH4+/K exchange between fluid and K-feldspar occurs simultaneously. However, there are
strong indications from Raman and IR spectroscopy that the incorporation of NH4+ into
K-feldspar is even faster than the consumption of the K-feldspar during the reaction, which is
already a very rapid reaction.
Ammonium has strong effects on fluid properties, element solubilities, and reaction
kinetics. It is not only determinant for the recycling of nitrogen, but also very important for
the composition of crustal and subduction zone fluids and the reactions occurring during the
subduction process.
Busigny, V. et al. (2003) Earth Plan. Sci. Lett., 215, 27-42.
Mingram, B. and Bräuer, K. (2001) Geochim Cosmochim. Acta, 65, 273-287.
Pöter, B. et al. (2004) Lithos, 74, 67-90.
Ridgway , J. et al. (1990) Appl. Geochem., 5, 475-489.
Watenphul, A. et al. (2009) Am. Mineral., 94, 283-292. |
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